The invention relates generally to a photomask and a method for fabricating the same and, more particularly, to an extreme ultraviolet (EUV) mask and a method for fabricating the same.
As the degree of integration of semiconductor devices increases and the design rule decreases, pattern sizes required for the devices are rapidly reduced. Therefore, as the wavelength of light used in a photolithographic apparatus becomes shorter and shorter, technologies to overcome a limit resolution in a photolithographic process for forming the pattern have been developed. For example, immersion lithography, Double Patterning Technology (DPT), and Extreme UltraViolet (EUV) lithography have been suggested.
The EUV lithography process uses light having a wavelength of 13.4 nm i.e. EUV, which is shorter than KrF or ArF light, to form a pattern less than 32 nm in size. A mask used in the EUV lithography includes a reflecting layer formed in a multilayer structure of a molybdenum layer and a silicon layer, on which an absorber layer pattern is formed as a shape of the pattern to be transferred onto a wafer. In order to obtain a reflection efficiency of about 67% in the EUV lithography process, the reflecting layer having a multilayer structure of molybdenum layer and silicon layer generally includes 40 to 50 layers. Increases in the size of the reflecting layer leads to increases in mask fabrication cost and increases in the probability of generating foreign substances during a deposition process. The EUV mask is formed in a shape such that the absorber layer pattern is projected above the reflecting layer. Light reflected from the EUV mask shows an intensity distribution in wherein the light intensity is highest in the center of the portion of the reflecting layer exposed by the absorber layer pattern and is lowered in the direction of the edge portion between the reflecting layer and the absorber layer pattern. Due to this light intensity distribution, the contrast of the transferred pattern is lowered. Therefore, the image of the pattern transferred onto the wafer becomes inaccurate, thereby resulting in poor exposure.